This site addresses three topics: fundamental physical constants, the International System of Units (SI), which is the modern metric system, and expressing the uncertainty of measurement results. Both essential information and background information are given for each topic, as described below.
Access to the
latest set of self-consistent values of
the basic constants of physics and chemistry recommended by CODATA (Committee
on Data for Science and Technology) for international use.
Access to the latest
conversion factors for energy
equivalents recommended by CODATA for international use.
bibliography giving the
citations of over 6400 published papers on the constants and related
aspects of establishing
Introduction to the
constants for nonexperts.
Links to selected
Introduction to the
essentials of the International System
of Units (SI), the modern metric system, adapted from NIST Special
Publications (SP) 330 and 811, which are based on internationally accepted
documents on the SI.
SI base units and
SI derived units. Includes a
graphical illustration of how the 22
SI derived units with special names and symbols are related to the seven SI
SI prefixes used to form decimal
multiples and submultiples of SI units. Includes a discussion of prefixes for binary multiples.
widely used units outside the SI.
rules and style conventions for proper SI
usage. Includes rules for correct typefaces
for symbols in scientific manuscripts.
Definitions of the SI base
of the base units: meter,
candela, including definitions of the
abbreviations CGPM, CIPM, and BIPM; and
brief history of the SI.
aspects of establishing
and maintaining the SI, including a list of the various
(Comités Consultatifs) of the International Committee for
Weights and Measures (CIPM, Comité International des Poids et
Links to related
sites that deal with units.
Bibliography for SI
units, including online
NIST publications on the SI.
SP 330, the
U.S. edition of the definitive reference on the SI published by the
International Bureau of
Weights and Measures (BIPM, Bureau International
des Poids et Mesures), which describes the SI and gives the various
historical recommendations concerning it.
SP 811, which not only
describes the SI but gives in detail the rules and style conventions for
printing and using units and for expressing values of quantities. Available
online in a PDF version (413 kB), or in an
HTML version, a printed copy may be
ordered at no charge.
Federal Register notice that gives the interpretation of the SI for
the United States. Available online.
which provides a brief history of U.S. weights and measures from the founding
of the country to about 1975.
About this reference.
Online: February 1998
Last update: January 2018
the essentials of the approach to
expressing measurement uncertainty that is rapidly becoming the single
internationally accepted method. Includes a
glossary of basic uncertainty terms.
type A evaluation of a standard uncertainty
component is based on any valid statistical method for treating data. A
type B evaluation of a standard uncertainty
component is usually based on scientific judgment using all of the relevant
combined standard uncertainty,
including simplified equation forms.
uncertainty and coverage factor.
that would be used in publications or correspondence.
U.S. perspectives on the effort that
has led to the near world-wide adoption of a single approach to the
expression of uncertainty in measurement.
Sources. The information on
evaluating and expressing measurement uncertainty within this reference is
adapted from NIST Technical Note 1297. Available online in a
PDF version (136 kB) or in an
html version, or a printed copy
may be ordered at no charge. This
publication is based on the comprehensive International Organization for
Standardization (ISO) Guide to the Expression of Uncertainty in
Measurement, or GUM (available from the BIPM website), available online in a PDF version. See also NIST Technical Note 1900, available online in a PDF version.
International Vocabulary of Basic and General Terms in Metrology, or
VIM, gives the definitions of many important terms relevant to the field of
measurement. Available for purchase.
Other: Frequently asked questions and Links to on-line unit conversions
About this reference. Feedback.
Online: February 1998 Last update: January 2018